# RSpec Mocks

rspec-mocks helps to control the context in a code example by letting you set known return
values, fake implementations of methods, and even set expectations that specific messages
are received by an object.

You can do these three things on test doubles that rspec-mocks creates for you on the fly, or
you can do them on objects that are part of your system.

## Messages and Methods

_Message_ and _method_ are metaphors that we use somewhat interchangeably, but they are
subtly different.  In Object Oriented Programming, objects communicate by sending
_messages_ to one another. When an object receives a message, it invokes a _method_ with the
same name as the message.

## Test Doubles

A test double is an object that stands in for another object in your system during a code
example. Use the `double` method, passing in an optional identifier, to create one:

```ruby
book = double("book")
```

Most of the time you will want some confidence that your doubles resemble an existing
object in your system. Verifying doubles are provided for this purpose. If the existing object
is available, they will prevent you from adding stubs and expectations for methods that do
not exist or that have invalid arguments.

```ruby
book = instance_double("Book", :pages => 250)
```

[Verifying doubles](./rspec-mocks/verifying-doubles) have some clever tricks to enable you to both test in isolation without your
dependencies loaded while still being able to validate them against real objects.

## Method Stubs

A method stub is an instruction to an object (real or test double) to return a
known value in response to a message:

```ruby
allow(die).to receive(:roll) { 3 }
```

This tells the `die` object to return the value `3` when it receives the `roll` message.

## Message Expectations

A message expectation is an expectation that an object should receive a specific message
during the course of a code example:

```ruby
describe Account do
  context "when closed" do
    it "logs an 'account closed' message" do
      logger = double()
      account = Account.new
      account.logger = logger

      expect(logger).to receive(:account_closed).with(account)

      account.close
    end
  end
end
```

This example specifies that the `account` object sends the `logger` the `account_closed`
message (with itself as an argument) when it receives the `close` message.

## Issues

The documentation for rspec-mocks is a work in progress. We'll be adding Cucumber
features over time, and clarifying existing ones. If you have specific features you'd like to see
added, find the existing documentation incomplete or confusing, or, better yet, wish to write
a missing Cucumber feature yourself, please [submit an issue](http://github.com/rspec/rspec-mocks/issues) or a [pull request](http://github.com/rspec/rspec-mocks).
